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Our solar system emerged from the Milky Way and another galaxy smashing into each other

By Elizabeth Rayne
Milky Way

Galaxies may look serene and otherworldy in all those NASA photos, but how they actually came into being was the total opposite.

Violence ruled the early universe. About 5.7 billion years ago, the nascent Milky Way collided with the Sagittarius galaxy. Astrophysicist Tomás Ruiz-Lara and his research team now believe this is how most of the stars in our galaxy emerged. There was already existing evidence for Sagittarius crashing into the disc of the Milky Way not just once, but three (possibly four) times over several billion years. After looking at data from ESA’s orbital space telescope Gaia, the team found out it is that these galactic mergers were much more intense than previously assumed — and the result was an onslaught of stars.

“Galaxy major mergers are thought to be one of the main factors triggering star formation in galaxies, and are predicted to have clear effects on their chemical evolution,” Ruiz-Lara and colleagues said in a study recently published in PLOS ONE, adding that “massive mergers play a critical rose in the formation of galaxies, including the Milky Way.”

Just how complex the Milky Way actually is remained unknown until Gaia shed more light on it through 3D maps of the entire galaxy and the positions of its stars. The problem with previous conclusions about our galaxy’s star formation history is that they either focused on just the solar system and its close surroundings or were unable to isolate the star formation events that made the Milky Way what it is now. These events happened about 5.7, 19 and 1 billion years ago (star formation decreased with every event as it does in massive spiral galaxies like ours). Some studies even missed the oldest and most immense burst of star births.

Gaia’s observations revealed that more stars formed in the thicker part of the Milky Way disc, though these were mostly older. Fewer but younger stars showed up in the thinner part of the disc. While stars have been emerging in the Milky Way throughout its existence, the collision 5.7 billion years ago produced the most stars in both areas of the disc, meaning that this was likely the most powerful star-forming event our galaxy has ever seen. This is the one that the Sun is thought to have spawned from. It was supposedly created when a monster nebula couldn’t handle its own gravity and collapsed under it. Now the Sun’s gravity holds the planets and moons of the solar system in place and influences geological conditions and phenomena.

Gaia even suggested a possible fourth star formation event that happened during the third event and lasted for the final 70 million years. That’s nothing in the cosmos.

If the Milky Way and Sagittarius had a Facebook relationship status, it would say “it’s complicated”. It was the gravitational pull of the Milky Way that ensnared Sagittarius and brought them close enough to crash. So how did encounters with Sagittarius lead to the formation of so many stars? Galactic mergers release gargantuan amounts of energy, sending a shock wave through the galaxy cluster and sending the gases within those galaxies into chaos. Such turbulence brings on a huge collapse that sets off the formation of cold, dense gaseous clouds that embryonic stars thrive in. Stars tend to multiply fast in the wake of such a phenomenon. They also tend to live fast and die young. Most of them perish after only a few million years, then explode into supernovae.

The force from supernova after supernova shoves huge amounts of gas out of the galaxy. Any surviving gas is consumed by star formation — not that there is enough left for another baby boom of stars. With the galaxy out of fuel, it just stagnates in space. “Pinpointing the causes of these star formation episodes will boost our knowledge of our own galaxy history, providing crucial information on the processes driving star formation on galactic scales,” Ruiz-Lara said.

Would this happen again? Unlikely. There is no proof of anything like this occurring in the late history of the Milky Way, so at least that’s one way doomsday will never happen.